Fluidized means for heat treatment of rubber



United States Patent [72] Inventors William Ferguson Watson Shrewsbury;William George Newell, Stirchley, near Wellington, England [21 1 Appl.No. 727,796 [22] Filed May 9, 1968 [45] Patented Nov. 17, i970 [73]Assignee Rubber and Plastics Research Association of Great BritainShretvshury, England a British Company [32] Priority May 10, 1967 [3 3]Great Britain [31] No.2],7l2/67 54] FLUIDIZED MEANS FOR HEAT TREATMENTOF RUBBER 10 Claims, 4 Drawing Figs. [52] U.S. Cl. 263/21, 34/57 [5|]lnt.Cl. .l F27b 15/00 [50] Field ofSeareh 34/10, 57;

' 263/21A, 40A; 165/104 [56] References (Iited UNITED STATES PATENTS2,326,163 8/1943 Patterson 263/21 3,250,52l 5/1966 Sergent 263/40A3,445,100 5/ i969 Bond 263/40A Primary Examiner-John J. Camby Attorney-Beveridge & DeGrandi Patented Nov. 17, 197 0 3,540,708

Sheet 1 012 FIG. 7.

g WILLIAM GEORGE IVE-WELL INVENTOR:

I WILLIAM FERGUSON WATSON Patented Nov. 17, 1970 Sheet INVENTOR: WILLIAMFERGUSON WATS N 4 WILLIAM csonee NEWE'LL BY 64%, swr-flwwlpolyethylene-propylene copolymeric elastomers. silicones and fluorinatedhydrocarbon elastomers and modifications thereof such as superiorprocessing natural rubber and partially vulcanised styrenebutadienerubber.

The continuous vulcanisation of continuous extruded rubber articles bythe fluidised bed process and the liquid curing medium process is nowwell established, and both methods are used in commercial production. Inthe liquid curing process, the rubber (containing additives includingcuring system) is extruded from an extruder, and passed into a bath ofliquid at a suitable temperature, generally in the range 180 to 250C. ltvulcanises in the bath'and is continuously hauled off at the end of thebath as a cured product. One difficulty with this process has been tofind a liquid suitable for use at such temperatures. Low melting pointmetal alloys have been used, but they are of unsuitable density and tendto leave a dross on the rubber which is difficult to remove, and the useof eutectic mixtures of molten sa'lts has therefore been preferred.These salts are, however, potentially dangerous, and personnel employedon liquid bath curing processes must be suitable protected by safetyclothing. The fluidised bed process, as proposed in British Pat. No.906,139, overcomes this difficulty by employing a bed of gas-fluidisedparticles of an inert material such as sand or glass which acts as aninert pseudoliquid".

Both of these processes, as conventionally operated, suffer from thedisadvantage that when vulcanisation takes place, any volatiles intherubber will tend to volatilise and expand rapidly, causingundesirable porosity in the rubber at the temperatures involved. Carecan be taken to ensure that volatile compoundingingredients e.g.volatile plasticisers, are not employed, but a certain amount ofmoisture always appears to be present in rubber, and air frequentlybecomes included during processing operations (e.g. in the extruder).While a lime/oil dispersion can be employed to absorb the moisture,leaving only a small amount ofincluded air to cause porosity, it wouldbe a considerable advantage over present methods to be able to preventthe air porosity from occurring, as this is unacceptable in applicationssuch as cables (see British Pat. No. 1,012,562) where a very solidrubber insulation is required to ensure electrical safety.

It is also extremely difficult to ensure that unwanted porosity isavoided in extruded articles which contain reinforcing fabrics, such ashose. In these cases air is entrapped in theinterstices of thereinforcing fabric, and expands rapidly under ambient pressure/hightemperature conditions causing porosity, blistering, and separation ofthe reinforcement from the rubber.

According to the present invention, a method of curing materials such asrubber comprises placing the liquid vulcanising or curing medium (whichmay be either a true liquid or a fluidised bed psuedoliquid") within aclosed vessel and applying to the vessel a pressure sufficient toprevent substantial expansion of the air, m'oisture or other volatilesor gases present in the article or material being cured.

The invention includes apparatus for carrying out this method, andcomprising:

1. An essentially closed vessel for holding a vulcanising or curingmedium; I

2. Means for maintaining within the vessel a positive gas pressuresufficient to prevent substantial expansion of air, moisture or othervolatiles or gases present in the material to be cured;

3. Entry and exit means for passing material to be cured through saidvessel; and

' clamped directly to the extruder, so that the extruder die 4. Means toprevent or minimise escape of gas under pressure from within the vesselat the points of entry and exit of the material to be cured.

In the case of a .vulcanising medium consisting of a hot liquid, forexample,-the pressure may be applied by a gas pressure line, via apressure reducing valve rated or set to compensate for losses of thepressurising gas through the entry and/or exit seals by which thecontinuous rubber article enters and leaves the pressurised vessel.Where the liquid is reactive, for example if it is a mixture of reactivesalts, it is preferred in the interests of safety to use an inert gas asthe pressurising gas.

In the case of a fluidised bed ,"pseudoliquid", the fluidising gas andthe pressurising gas may conveniently be the same gas. In this case itis essential that the gas continues to flow through the fluidisingapparatus at the appropriate rate to maintain the fluidisation, so thepressure may be controlled by placing an adjustable pressure reliefvalve in a gas exit from the apparatus. This valve can be adjusted toblow off". at a predetermined required. pressure. For example, afluidised bed fluidised, into the part ofthe closed vessel above themedium,

and finally leaves the vessel by the pressure, relief valve.

In general, the gas used to fluidise and pressurise the bed may be airor steam or an inert gas. Steam has been considered to be desirable onlybecause in the superheated condition it is a cheap, readily available,dry inert gas. For a large proportion of curable materials air would bemore satisfactory if available at high volumes and pressures.

As the closed vessel containing'the vulcanising medium is maintainedinternally'at a pressure in excess of ambient pressure, for example, apressure between 20 andp.s.i., it is necessary to pass the rubberextrusion to be vulcanised through entry and exit sealing glands toprevent excessive loss of the pressurising gas. The sealing isparticularly easy in the case of hose and cable extrusions since theseare of circular cross section and can be sealed, e.g. by seals of thetype conventionally used in sealing cables atthe exit from high pressuresteam tubes as conventionally used for curing cables. in the caseofhose, the sealing is more effective if the hose is supportedinternally by a flexible mandrel'(as described, for example, in US. Pat.No. 2,974,713). a

it is more difficult to arrangeentry sealing than exit sealing, sincethe rubber before vulcanisation is soft and liable to flow. While thisis no great problem with supported and reinforced extrusions of simple(e.g. circular) cross section, it is nevertheless an advantage to beable to dispense with the entry sealing glandWith more complicated crosssections, such as weather stripping for building and car applications,sealing the soft unvulcanised article into the pressurised vessel isdifficult. ltis therefore proposed that the pressurised. vessel may bedelivers the extrusion directly. into the vessel, and an entry seal isunnecessary. To arrange an exit seal is not difficult, since at the exitthe rubber is cured, resilient, and amenable to sealing.

The inventionwill now be further described by way of example, withreference to the accompanying drawings which show a form of apparatuswhich may be used for the continuous vulcanisation of, rubber extrusionsusinga fluidised bed curing medium, and operating at pressures of about20 to 80 p.s.i.

' F 1G. 1 shows a side elevation of such an apparatus;

FIG. 2 shows a cross section of the sameapparatus on the line 2-2 ofFIG. 1;

FIG. 3 shows the particularly advantageous circulation pattern offluidised particles in such an apparatus; and

FIG. 4 is a modification of part of the apparatus shown in FIG. 1.

Referring to FIGS. 1 and 2, a closed vessel in the form of a main tube 1in unit lengths is supported on wheeled stands 2. The stands 2 alsosupport unit lengths of a feed pipe 3 which is connected by branch pipes4 through fluidising inlet ports 5 in the base of the tube 1 with theinterior of tubular porous tiles 6 mounted within the tube 1. The tube 1is also provided with internal heaters 7 and external muff heaters 8.The upper part of the main tube 1 communicates with an exhaust pipe 9which terminates in a pressure relief valve 10 and is supported on thetube 1. The pipe 9 is also in unit lengths.

Each wheeled stand 2 and the length of pipe 1 which itcarries'constitutes a unit and any number of these units may be coupledtogether as shown in the drawings to form a continuous length of maintube 1. For this purpose, the main tube lengths 1 are bolted togetherwith suitable sealing and the lengths of feed pipe 3 are likewise boltedand sealed. The lengths of exhaust tube 9 are coupled together by meansof a flexible bellows 11 to allow for misalignments, and differentialexpansions when the apparatus is heated. The wheeled stands may alsosupport temperature control units. The main tube 1 is also provided withfilling and viewing ports 12, and with drain plugs 13 for maintenancepurposes. The ends of the main tube 1 are provided with telescopic seals14 to allow rubber extrudate or other material to be cured to pass intoand out ofthe main tube 1.

On its inlet side, the feed pipe 3 includes a pressure reducing valve 15which is followed by a superheater 16.

In use, the main tube 1 is filled to approximately 2 inches above thecentreline with the ballotini (tiny glass spheres) to be fluidised, thetube being approximately 8 inches in diameter. The fluidising steam isderived from the normal high pressure factory shop line and is firstreduced in pressure by the valve 15 and then superheated to thetemperature required at 16. Typical operating temperatures and pressuresin the main tube 1 are 180 to 250C. at to 50 psi. The steam is fed toeach bed section by. feed pipe 3 through the fluidising inlet ports 5into the centre of the porous tiles 6. Steam escapes from the poroustiles into the ballotini, the steam velocity being high enough to ensurethat the ballotini is fluidised.

The fluidised ballotini is heated by the internal heaters 7 and externalheat losses are prevented by the external muff heaters 8. The internalheaters 7 may also act as baffles and are preferably situated above theporous tiles 6 and offset from the centreline of the tiles. Thispreferred construction causes a recirculating pattern of movement to beset up in the ballotini as shown in FIG. 3. There are two mainadvantages in this recirculating pattern. Firstly, it gives very goodheat scavenging from the heater surface and distributes this heatquickly and evenly throughout the ballotini. Secondly, it gives anatural tendency for any rubber extrudate floating in the bed to traveldown the centre, thus tending to prevent the snaking that occurs in theordinary fluid bed using plain porous tiles and a rectangularcross-sectional bed. The extrudate, while still floating, ismade tositlower in the fluidised ballotini, thus giving better and more uniformheating.

The steam which has passed through the ballotini passes through theexhaust pipe 9 to the pressure relief valve .10 and is then eitherexhausted to atmosphere or cooled and passed into the normal condensateline. The back pressure in the main tube is controlled by the pressurerelief valve. In places of steam, air may be used.

In order to reduce the volume of fluidising gas required to be suppliedto the apparatus provision may be made to recirculate the gas. Gas maybe taken from the exhaust pipe 9-via a suitable filter to a rotaryblower capable of producing a pressure differential greater than thepressure drop across the system from feed pipe 3 to exhaust pipe 9. Thusthe large volume flow of gas required to cause adequate fluidisation maybe provided by the rotary blower while the pressure and losses via sealsand relief valve 10 would be provided via the feed pipe 3 from thenormal factory air or steam line.

The rubber extrudate or other material to be cured passes in and out ofthe bed through the telescopic seals 14, the design of which depends onthe nature of the extrudate and the internal pressure in the main tube.They are telescopic in part in order to facilitate the initial feedingof extrudate through the bed. The feed end may be bolted directly to theextruder, thus eliminating sealing problems at the point where theextrudate is soft and easily distorted.

For circular cross section extrusions such as hose or cable one type ofseal may take the form of a tube 17 of rubber or suitable resilientmaterial as shown in FIG. 4. This may be supported such that thediameter of the hole through the tube is greater at either end than atthe centre, the centre diameter being smaller than the extrusion to beproduced. The whole tube may be mounted in a case 18 with the ends ofthe tube sealed to the case and the cavity between the case and theouter wall of the rubber tube filled with liquid (eig. glycerine via aninlet l9)v The volume of this cavity may be varied by means of a plunger20 slidable in the casing so that the centre diameter of the sleeve willalso be varied and can be adjusted to be a close fit on the extrusionthus controlling the quantity of gas escaping through the seal.

The position of this plunger may be adjusted manually or may becontrolled by the pressure in the bed (vessel 1) via a pipe 21 providedwith a regulator 22 as shown in FIG. 4. If therefore the gap between theseal and the extrusion became too large the pressure in the bed wouldfall. This fall of pressure may be made to move the plunger in such adirection as to reduce the centre diameter of the rubber tube thusreducing the gap between seal and extrusion.

Tubular sections have been used throughout as being the best shapesuitable for a pressurised vessel.

The use of two tubular porous tile units for fluidising in place of thestandard fluid bed flat tile offers certain advantages. There are lessproblems involved in sealing the tiles together and more reliableperformance can be expected. The recirculating pattern obtained givesimproved heat distribution and induces the rubber to travelup the centreof the bed. Finally, the tiles are easily removable for maintenance.

In general, the fluidising apparatus of the invention comprises a closedvessel for the fluidising medium, rneans to supply a fluidising gas tothe fluidising medium, means to withdraw from the closed vesselfluidising gas after'it has passed through the fluidising medium andmeans to maintain a predetermined pressure within the closed vessel.

While the invention has been described primarily with respect to thecuring of rubber, it will be appreciated that it may also be used forcuring other polymers which can be subjected to heat treatment to causea cross linking reaction, such as curable plastics. A particular exampleof a curable plastic which maybe employed is polyethylene containing aperoxide or other cross linking agent.

We claim:

1. Apparatus for curing curable elastomeric materials comprising:

1. An essentially closed vessel for holding a fluidisable bed curingmedium; i

2. Means arranged to fluidise said bed when in said vessel, said meansbeing also arranged to maintain within the vessel a positive gaspressure sufficient to prevent substantial expansion of volatiles orgases present in the elastomeric material to be cured;

3. Entry and exit means for passing material to be cured continuouslythrough said vessel; and

4. Means to prevent or minimize escape of gas under pres sure fromwithin the vessel at the points ofentry and exit I of the material to becured.

2. Apparatus as claimed in claim 1 in which the entry and exit meanscomprise seals.

3. Apparatus as claimed in claim 1 in which the means for maintainingthe positive pressure within the vessel comprise a gas pressure line forsupplying pressurising gas to said vessel, the gas pressure lineincluding a pressure reducing valve set to compensate for losses of thepressurising gas.

4. Apparatus as claimed in claim 3 in which an adjustable pressurerelief valve is provided in a gas exit from the closed vessel.

5. Apparatus as claimed in claim l in which means are provided to heatthe contents of the closed vessel.

6. Apparatus as claimed in claim 3 in which the gas pressure linecommunicates with the interior of the closed vessel through a porousmaterial on or around which material to b fluidised may be disposed.

7. Apparatus as claimed in claim 6 in which the porous material takesthe form of at least one tubular porous tile.

8. Apparatus as claimed in claim 1 in which the entry or exit

